The present invention relates generally to wireless telecommunication technology, and more particularly to an apparatus for detecting uplink ranging codes in a wireless communication system.
A wireless communication system is typically consisted of a number of base stations deployed among geographical areas for transmitting and receiving signals to and from a plurality of mobile stations. The mobile station receives downlink messages from the base station, and then transmits a set of uplink ranging codes to the base station through a ranging channel, typically a distributed sub-radio channel, in order to establish a link there between. The ranging code is a particular set pseudo-random code for the purpose of power adjustment information, bandwidth requirement, and time and frequency offsets between the signal timing of the mobile station and a reference point predetermined by the base station. Since in a wireless communication system, a base station may communicate with a plurality of mobile stations at the same time, it is common that the base station may receive various sets of ranging codes simultaneously from various mobile stations seeking to establish communication links.
When a base station receives data from a mobile station via uplink channels, it needs to extract ranging codes from them in order to determine information, such as time/frequency offsets and power adjustment information. Conventionally, the base station would compare the received data with an entire ranging code book, a set of sequences from which ranging codes are selected, to identify the ranging codes transmitted from the mobile station for further estimation of time/frequency offsets and power adjustment information.
FIG. 1 illustrates a block diagram showing a conventional apparatus 100 implemented in a base station for detecting uplink ranging codes carried by data received from one or more mobile stations. Each block in the drawing represents one or more particular functions that may be performed through hardware or software implementations. A Fast Fourier Transform (FFT) unit 102 transforms the data received from the mobile station with the FFT algorithm. A cross-correlation unit 104 receives outputs from the FFT unit 102 and possible ranging codes from a ranging code book to determine the correlation there between. An Inverse Fast Fourier Transform (IFFT) unit 106 receives outputs from the cross-correlation unit 104 and transforms them with the IFFT algorithm. A ranging code detection and time offset estimation unit 108 receives outputs from the IFFT unit 106 to detect the ranging codes of the received data, and estimate time offset from the detected ranging codes.
One drawback of the conventional apparatus is that its data processing is complex and resource-consuming when the ranging code book contains a large number of ranging codes, since all the outputs from the cross-correlation unit 104 must be processed by the IFFT unit 106. Another drawback is that the conventional apparatus may not be able to estimate time offset accurately. In the prior art, the time offset is estimated by the highest peak location of the received data signal in a process domain. Because the ranging channel is a distributed sub-radio channel, several peaks would appear on the time line in the process domain. Ideally, the location of the highest (the middle) peak represents the time offset. However, in reality, the location of the highest peak may be away from its noise-free location, especially in a low over-sampling environment. As a result, the time offset estimated by the highest peak position may not represent the true time offset accurately.
Thus, what is needed is an apparatus for detecting uplink ranging codes that balances data processing complexity and reduces inaccuracy in estimating time offsets.